Digital communication is often accomplished by phase modulation of a carrier with either 0.degree. or 180.degree. of phase shift corresponding to a logic state of 0 or 1. Frequently, two such communication channels utilizing carriers which are phase shifted from each other by 90.degree. are employed. The two channels are summed together to give a resultant carrier which experiences phase modulation of .+-.45.degree. or .+-.135.degree., these four possible phase states representing the two logic states in each of the two channels. The resultant phase-modulated carrier is referred to as being modulated by quaternary phase-shift keying (QPSK). Such a system is described in the book entitled "Data Transmission" by William R. Bennett and James R. Davey which was published by McGraw-Hill Book Company in 1965.
A QPSK modulated carrier is frequently transmitted via a microwave radio link at a carrier frequency of, for example, 10 gigahertz (gHz). In order to provide suitable power of radiated energy in a microwave transmission link, an injection-locked oscillator is often employed. An injection-locked oscillator type of amplifier is convenient to use because of its physically small size, it being composed of, essentially, a diode affixed within a suitably tuned portion of a waveguide with a voltage applied across the diode from an external source of electric power.
A problem arises when the injection-locked oscillator type of amplifier is employed with a QPSK modulated carrier, the problem being that a quantum phase shift of 180.degree. frequently occurs. Such an occurrence is characterized in a typical communication system by a momentary drop in the amplitude of the modulated carrier with the result that the oscillation of the amplifier loses lock with respect to the modulated carrier. Ideally, the injection-locked oscillator is tuned to oscillate at the carrier frequency of the signal to be amplified as this relationship provides for maximum efficiency of the amplification process. Unfortunately, this frequency relationship intensifies the foregoing problem. Attempts to alleviate the problem by offsetting the oscillation frequency from that of the signal to be amplified degrades the quality of the amplification in that there is a loss of phase coherence between the input and output of the amplifier with attendant phase errors. As a result, the usefulness of the injection-locked oscillator is greatly reduced in the situation where it is desired to transmit accurate data via a QPSK modulated carrier.